Considerable time-lags in the range of motor starting currents can be achieved in a great number of different ways. We have found, however, that almost all of these ways are incompatible with the requirement of minimizing peak let-through currents and clearing I.sup.2 .multidot.t values.
Simple solder joint fusible elements, or elements having overlays of solder, i.e. fusible elements which rely solely on the so-called M-effect, are hardly capable of producing considerable time-lags in the motor starting current range.
Fusible elements such as those disclosed in U.S. Pat. No. 2,321,711; June 15, 1943 to E. H. Taylor for FUSIBLE ELECTRIC PROTECTIVE DEVICE, are capable of producing considerable time-lags in the low current range, but incapable of meeting the requirements of minimal peak let-through currents and clearing I.sup.2 .multidot.t values, mainly because according to this patent a substantial portion of the total length of the fuse is occupied by special low current time-lag interrupting means which do not contribute in any way to high fault current interruption.
The only fuse which meets long time-lag requirements in the motor starting current range, and whose fusible elements can be manipulated or changed in such a way as to minimize peak let-through currents and clearing I.sup.2 .multidot.t is that disclosed in U.S. Pat. No. 3,189,712; June 15, 1965 for HIGH INTERRUPTING CAPACITY FUSE. The fuse disclosed in that patent is capable of achieving in the motor starting current range satisfactory time-lags. The fuse links of this fuse extend along the preponderant length of the casing and can, therefore, be tailored to meet let-through current and clearing I.sup.2 .multidot.t requirements. According to this invention the ratio of the diameters of the transverse lines of circular perforations must be in the order of 6:1 and the cross-sectional area of each of the solid metal current paths between perforations must be in the order of 0.00005 to 0.000075 square inches. These are critical values. By following them, the peak let-through amperage can be reduced to smaller values than specified in Underwriters Standards RK5, and to meet the requirements of Underwriter Standards RK1. The above geometry results, however, in a reduction of the length of the current path between the circular perforations, and hence in a reduction of the arc voltage generated. This, in turn, results in an increase of the clearing I.sup.2 .multidot.t values above those permitted by the Underwriter Standards. This limitation can, however, be remedied by increasing the number of transverse lines of circular perforations. Thus a fuse according to U.S. Pat. No. 3,189,712 meeting Underwriter Standard RK5 and designed for a circuit voltage of 250 volts has three transverse lines of circular perforations, while the above dimensions in regard to the size of the perforations and that of the intervening portions call for the presence of an additional transverse line perforation, i.e. a total of four lines. Similarly, a fuse according to U.S. Pat. No. 3,189,712 meeting Underwriter Standard RK5 and designed for a circuit voltage of 600 volts had five transverse lines of circular perforations, but the reduction of arc voltage resulting from the above perforation size and inter-perforation metal bridge size with its attendent reduction of arc voltage calls for six rather than five transverse lines of circular perforations.